1. Field of the Invention
The invention relates to internal combustion engines, and in particular, to homogenous charge compression ignition (HCCI) engines.
2. Description of the Related Art
Emission control standards for internal combustion engines have tended to become more stringent over time. The sorts of emissions to be controlled tend to fall into at least four broad categories: unburned hydrocarbons, carbon monoxide particulates, and oxides of nitrogen (NOx).
Unburned hydrocarbons and carbon monoxide tend to be products of incomplete combustion of a hydrocarbon fuel. Each atom of carbon in the fuel requires two atoms of oxygen with which to combine for complete combustion. If each carbon atom finds two oxygen atoms with which to combine, carbon dioxide is formed. The remaining hydrogen atoms combine with two oxygen atoms apiece to form water.
If only one atom of oxygen is available to combine with a carbon atom, on the other hand, carbon monoxide is formed. If no oxygen is available, hydrocarbons are left unburned. Thus, reduction of unburned hydrocarbons and carbon monoxide depends on the provision of adequate oxygen during combustion to oxidize the carbon atoms completely.
Compression ignition engines are generally run with an excess of air over the stoichiometric ratio to ensure adequate oxygen supplies are available for combustion. Particulates tend to be produced by reactions that are close to stoichiometric as well, so the availability of an excess of oxygen over stoichiometric may reduce those as well.
Nitrogen is a major component of air. Nitrogen is inert at standard temperature and pressure. Nitrogen becomes reactive, however, at heightened temperatures and pressures. The heat associated with high temperatures thus serves as a catalyst for nitrogen. High temperatures tend to be associated with complete combustion, since combustion is exothermic. The high temperatures associated with complete combustion may thus cause nitrogen to react with oxygen and form oxides of nitrogen.
One way to control the production of oxides of nitrogen is to limit the combustion chamber temperatures reached during combustion. Since heat associated with high combustion temperatures serves as a catalyst for nitrogen, reducing the peak combustion chamber temperature may reduce the reactivity of nitrogen. Since reducing the peak temperature ameliorates one of the conditions necessary for the production of oxides of nitrogen, there may be a consequent reduction in the quantity of oxides of nitrogen that are produced.
Fuel is injected, on the average, into the center of a combustion chamber in a conventional compression-ignition engine. The fuel is injected after the incoming air charge has been compressed sufficiently to ignite the fuel, and thus the fuel burns almost immediately. Since the fuel burns almost immediately, it has relatively little time to distribute itself evenly about the combustion chamber. Since the fuel is not distributed evenly during combustion, but rather is localized, a large quantity of fuel is available in a small volume to support combustion. Since a large quantity of fuel is available to support combustion, combustion proceeds for a relatively long period of time, and high temperatures of combustion are able to develop.
With HCCI engines, on the other hand, fuel is injected during the compression stroke, while the incoming air charge is being compressed. The combustion event occurs once the air charge has been compressed enough to raise its temperature to the kindling temperature of the fuel. The fuel thus has some time to propagate throughout the volume of the combustion chamber before combustion takes place. Furthermore, the swirling and tumbling of the air charge during compression may promote distribution of the fuel before the combustion event takes place.
Since the fuel has time to propagate throughout the combustion chamber volume before ignition takes place, ignition may occur simultaneously throughout the combustion chamber volume. This may, for example, allow the combustion process to rely less on propagation of a flame front to burn the fuel than would be the case with conventional compression ignition.
The combustion rate may consequently be higher, since there will be no delay associated with waiting for a flame front to progress across the combustion chamber. This may allow a more dilute mixture of air and fuel to be used. This may also allow the peak temperature to be reduced, thereby reducing formation of oxides of nitrogen, since the fuel burns completely in less time than it would take for comparable localized combustion.
Since the combustion event in an HCCI engine occurs once the air charge has been compressed enough to raise its temperature to the kindling temperature of the fuel, the timing of the combustion event may vary somewhat from cycle to cycle. The timing of the combustion event in an HCCI engine may thus be relatively more difficult to control than the timing of a combustion event in a conventional compression ignition engine. Variability of combustion timing may manifest itself as roughness or pre-detonation, also known as “knocking.” It would be desirable for the timing of the combustion event in an HCCI engine to be more precisely controllable.